1. Field of the Invention
This invention relates generally to rotary wing aircraft, and particularly to rotor blades used on rotary wing aircraft.
2. Brief Description of the Related Art
Recent improvements in rotary wing aircraft rotors have been developed primarily for helicopter applications in which the rotor is powered full time, as opposed to gyrocopters or gyroplanes in which the rotor is not powered or is powered only prior to takeoff. While the leading edge and trailing edge extensions of the present invention are advantageous for both helicopters and gyroplanes, some of the requirements of the two applications are different. For example, it is desirable for gyroplanes to have jump takeoff capability, in which the rotor is spun up on the ground to a high rotational speed (much higher than is used for takeoff in helicopters) with zero blade angle of attack, then vertical takeoff is performed by increasing the blade angle of attack. The high rotational rate required for jump takeoff requires a rotor with higher in-plane stiffness than is required for helicopters, since the stiffness required to maintain a natural frequency higher than the maximum rotation rate (required in two-bladed rotors) increases with the square of the rotation rate.
At high aircraft forward speed, inboard portions of the retreating blade are stalled and provide little lift, so only the tip of the retreating blade will be providing lift. Since the advancing and retreating blades must provide equal lift moments around the rotor head, the advancing blade can only provide as much lift moment as the retreating blade. Rotor RPM of the helicopter is typically selected as a compromise between desiring a slow RPM to prevent entering compressibility in the advancing blade while desiring an increased RPM to prevent or minimize retreating blade stall.
Gyroplanes or other rotary wing aircraft designs having an alternative lift producing mechanism utilized during cruise flight, thus are not as concerned with retreating blade stall. Instead, the gyroplane type aircraft are concerned with reducing drag caused by the rotating rotor. For example, if the pilot wishes to accelerate rather than climb, as the aircraft speed increases, the pilot can level the rotor tip path plane. This reduces the rotor plane of rotation relative to the airstream, reduces the rotor disk angle of attack, and slows the rotor down. Once the rotor has been sufficiently unloaded by providing lift with the alternate source, such as fixed wings, and providing thrust with an alternate source, such as a propeller or jet engine, the rotor blades can maintain lift moment equilibrium about the hub with only the rotor flapping.
Rotor flapping is the mechanism by which the advancing blade and retreating blades can produce the same lift moments. In order to work, the blades must be free to move up and down. This free flapping allows the advancing blade, which has more lift due to a higher velocity across it than the retreating blade, to rise or flap up. As the advancing blade rises, the resultant (vector sum of horizontal and vertical air velocities) flow angle across the blade (angle of attack) drops and reduces its lift. The faster the advancing blade rises, the more the resultant angle of attack is reduced and the more its lift drops. The opposite occurs on the retreating blade. As the advancing blade goes up, the retreating blade drops since the blades are tied together and because the retreating blade is not producing as much lift as the advancing blade. As the retreating blade drops (flaps), the resulting angle of airflow across the blade goes up and increases its lift. The faster the retreating blade drops, the more its angle of attack is increased and the more its lift increases. This characteristic whereby the lift on the retreating blade increases as the blade drops works whether the air flows from leading edge to trailing edge or from the trailing edge to the leading edge. The flapping automatically increases until the vertical velocity component changes the angle of attack on both the advancing and retreating blades until they both have the same lift.
As the rotor RPM slows down, the centrifugal force decreases until at some point there would not be enough centrifugal force to keep relatively soft and flexible rotor blades stable. To allow the rotor to be slowed down as much as practical, weight is added to the blade tips.
Rotor blade tip weights have been used in gyroplane type aircraft in order to increase rotor inertia to minimize the required rotor RPM during pre-takeoff. However, increasing the weight placed in the rotor blade in order to additionally reduce the rotor RPM requires an increase in the in-plane stiffness necessary to maintain a natural frequency higher than the maximum rotation rate. The horsepower requirement necessary to spin a rotor of a gyroplane type aircraft is typically a function of the cube of the rotor RPM ratio. That is, the applicant has recognized that a three-fold reduction in rotor RPM from 300 RPM to 100 RPM will reduce the horsepower required to spin the rotor at 100 RPM to that of 1/27 that required at 300 RPM. This reduction in the RPM results in higher mu values, mu being the ratio of forward speed of aircraft to tip speed of rotor relative to the speed of an aircraft. The prior art, however, has not shown a method or device to provide a rotor with sufficient structural integrity and stability to reduce the rotor RPM sufficiently to obtain a mu value greater than 0.87. Thus, the applicant has recognized the need for a rotor blade design that can maximize stability in the overall rotor as a whole, while providing sufficient structural support to support blade tip weights to achieve a mu value of at least 1.0.